1. Field of the Disclosure
The present disclosure relates in general to marine riser tensioners and, in particular, to a push up tensioner assembly that accommodates riser tilt.
2. Brief Description of Related Art
Offshore production platforms must support production risers from oil or gas wells that extend to the platform from subsea wells. For subsea completions in deep water that require the use of floating platforms, such as tension leg platforms (TLPs) or semi-submersible platforms, supporting risers presents significant problems. In TLPs, tension legs extend from the platform down to an anchor located at the sea floor. The tension legs are relatively inelastic, meaning that much of the vertical motion of the platform is eliminated. TLPs allow for location of the wellhead assembly on the surface rather than on the sea floor. A riser will typically extend from the wellhead assembly down to the sea floor. This setup allows for simpler well completion and better control of production. However, in TLPs the riser may tilt from the vertical relative to the TLP. The amount of riser tilt from the vertical is not static and varies with time during operation.
Because floating platforms move under the influence of waves, wind, and current and are subjected to various forces, the riser tensioning mechanism is typically designed to permit the platform to move relative to the riser. The riser tensioning mechanism also usually maintains the riser in tension so that the entire weight of the riser is not transferred to the wellhead, and the riser does not collapse under its own weight. The tensioning mechanism therefore exerts a continuous tensional force on the riser that is maintained within a narrow tolerance.
Push up tensioners generally have operational, reliability, and safety advantages over conventional pull-up tensioning systems. For example, a push up tensioner accommodates higher loads in a smaller space over other types of tensioners. This is in part because push up tensioners use a more efficient piston end and do not require a tension pulling device at the end connection. In addition, use of a push-up tensioner can minimize the corrosive effects of the salt-water environment in which they operate. This is because the high pressure seals of the push-up tensioner are not located adjacent to the atmosphere and are isolated from caustic fluids and debris.
However, for TLP applications, where low loads and stroke lengths are combined with the need to accommodate angular offset of the riser, the current style of push-up tensioner has a cost premium relative to the conventional pull-up system. Current push up systems become relatively economical when stroke range approaches or exceed about 10 ft., or production riser loads exceed about 1000 kips. Current TLP push-up tensioners typically include hydro-pneumatic assemblies that are mounted at an inwardly tilted angle from the deck into a tension load ring. The connection of the cylinder to the deck and load ring often requires a costly mounting system that allows angular offset between the cylinder and deck/load ring. While this configuration works well for TLP riser systems that require high loads and stroke lengths, a large portion of the cost of the system is due to the mounting system and such cost is substantially similar for all such systems, no matter what the stroke length or load requirement might be. Therefore for low stroke length or load requirements, current push up systems are not cost competitive compared to more conventional tensioner systems.